Sounds like that's what's happened to me, as the AC was on for nigh on 2 hours at full blast due to the summer weather we were having. I've read that the A/C is constantly on, but if the gas has leaked out or the valve is broken the motor is still running, and hence the whining noise alongside the engine. Anyway will see what Mercedes have to say when i get down there. Did they try to say it wasn't covered under warranty? This is the first time i will be claiming on it and hopefully it will go smoothly.Had a problem with mine a year or two ago.
Apparently the condenser had become "porous" and leaked all of the gases out.
Was replaced under warranty and has been fine since. Mine wasn't the first time my dealer had seen this issue!